The stock price for Amgen (down $2.56 to $61.70, Charts), the world's leading biotech in terms of sales, slipped more than 3 percent after the bell, adding to a 14 percent decline since mid-January.

"This is almost panic selling on the litany of issues that seem to be hounding the company," said Jim Reddoch, analyst for Friedman, Billings, Ramsey & Co.

Thursday's price drop follows an annual report filing from Amgen, where the company said it received an "informal inquiry" from the Securities and Exchange Commission about the way it handled a Danish study regarding its top-selling anemia treatment Aranesp last year.

"We're working with [the SEC] as we always do in these things," said Amgen spokesman David Polk to CNNMoney.com.

An SEC spokesman refused to comment.

Also, Amgen said it's in safety discussions with the Food and Drug Administration over Aranesp, Epogen and other treatments for anemia, a commonly occurring disease affecting the production of red blood cells. These drugs are typically used with chemotherapy, which can cause anemia, and for kidney disease patients, including those undergoing dialysis. The FDA has scheduled a panel to discuss anemia drug safety on May 10, according to Amgen.

"Is this going to be an immaterial hit, or a 20 percent hit?" said UBS analyst Maged Shenouda, referring to the potential impact on Amgen revenues. "I don't think that anyone has an answer for that, but the stock continues to sell off."

Shenouda believes that FDA discussions could lead to a "black box" warning for some of the uses of these drugs, referring to the agency's most serious warning. The analyst said that Amgen is conducting a safety study on its anemia drugs, and the results could help or hinder the company.

"[Amgen needs to] provide a compelling case that these products are safe and effective as per their label," said Shenouda.

"The biggest issue with Amgen" is that "half of its revenue comes from one product type," said Reddoch of Friedman, Billings, Ramsey.

Also, Amgen could face competition from the Swiss drug giant Roche, which is trying to get its experimental anemia drug CERA approved by the FDA this year.

Geoffrey Porges, analyst for Sanford C. Bernstein, has lowered Amgen's sales and earnings estimates because of problems with Aranesp and Epogen. Porges lowered his 2007 sales estimate by $126 million, or 2 percent, and earnings per share by 4 cents, to $4.34.

Porges says that the problems surrounding these drugs will take a gradually bigger bite out of his sales estimates. He has lowered his sales estimates by more than 10 percent from 2009 through 2013.

Amgen, based in Thousand Oaks, Calif., is the world's largest biotech in terms of sales. Genentech (down $0.82 to $83.59, Charts), based in South San Francisco, is number 2.

The analysts quoted in this story do not own shares of Amgen stock, but UBS and FBR have conducted investment banking services for the company in the past 12 months, and FBR co-managed a public offering of the companies' securities.